Free Astronomy Magazine September-October 2021

21 SEPTEMBER-OCTOBER 2021 Pedro Bernardinelli and Gary Bern- stein, of the University of Pennsyl- vania, found the comet — named Comet Bernardinelli-Bernstein (with the designation C/2014 UN 271 ) — hidden among data collected by the 570-megapixel Dark Energy Camera (DECam) mounted on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile. The analysis of data from the Dark Energy Survey is sup- ported by the Department of En- ergy (DOE) and the National Science Foundation (NSF), and the DECam science archive is curated by the Community Science and Data Center (CSDC) at NSF’s NOIRLab. CTIO and CSDC are Programs of NOIRLab. One of the highest-performance, wide-field CCD imagers in the world, DECam was designed specif- ically for the DES and operated by the DOE and NSF between 2013 and 2019. DECam was funded by the DOE and was built and tested at DOE’s Fermilab. At present DECam is used for programs covering a T his illustration shows the distant Comet Bernardinelli-Bernstein as it might look in the outer Solar Sys- tem. Comet Bernardinelli-Bernstein is estimated to be about 1,000 times more massive than a typical comet, making it arguably the largest comet discovered in modern times. It has an extremely elongated orbit, journey- ing inward from the distant Oort Cloud over millions of years. It is the most distant comet to be discovered on its incoming path. [NOIRLab/NSF/ AURA/J. da Silva (Spaceengine)] huge range of science. DES was tasked with mapping 300 million galaxies across a 5,000-square-de- gree area of the night sky, but dur- ing its six years of observations it also observed many comets and trans-Neptunian objects passing through the surveyed field. A trans- Neptunian object, or TNO, is an icy body that resides in our Solar Sys- tem beyond the orbit of Neptune. Bernardinelli and Bernstein used 15–20 million CPU hours at the Na- tional Center for Supercomputing Applications and Fermilab, employ- ing sophisticated identification and tracking algorithms to identify over 800 individual TNOs from among the more than 16 billion individual sources detected in 80,000 expo- sures taken as part of the DES. Thirty-two of those detections be- longed to one object in particular — C/2014 UN 271 . Comets are icy bodies that evapo- rate as they approach the warmth of the Sun, growing their coma and tails. The DES images of the object in 2014–2018 did not show a typical comet tail, but within a day of the announcement of its discovery via the Minor Planet Cen- ter, astronomers using the Las Cum- bres Observatory network took fresh images of Comet Bernardi- nelli-Bernstein which revealed that it has grown a coma in the past 3 years, making it officially a comet.